class_docstr: Class for the Hitting Set Algorithm to obtain a approximate minimal set of backdoor variables
  to condition on for each node pair.
docstr: Obtain number of backdoor paths between a node pair.
docstr_tok: ["Obtain","number","of","backdoor","paths","between","a","node","pair","."]
code: def num_sets(self):
      """
      Obtain number of backdoor paths between a node pair.
      """
      return len(self._list_of_sets)
  
imports: import networkx
  from dowhy.utils.graph_operations import adjacency_matrix_to_adjacency_list
label_desc: Use image node_id 2 for calling the HittingSetAlgorithm obj's underlying member method code with example usage: obj.num_sets() and returns: len